An optical circulator is a device that has at least three ports for accepting optical fibers. Light that enters the circulator through the first port exits through the second port; light that enters through the second port exits through the third. The optical circulator is an inherently non-reciprocal device, since if light enters through the first port it exits through the second, but if that light is subsequently reflected back into the second port, it does not retrace its path back to the first port, but exits through the third port instead.
Circulators are necessary, for example, to use the same fiber for both receiving and transmitting data. The first port may be connected to a data transmitter, and the second port to a long distance optical fiber. In that case, data can be sent from the transmitter to the fiber. At the same time, incoming optical data from the long distance fiber enters the circulator through the second port and is directed to the third port where a receiver may be connected.
One prior art optical circulator is described in U.S. Pat. No. 4,650,289 by Kuwahara; see FIG. 1. In this circulator, the labels A, B, and C correspond to the first, second, and third ports described above (port D need not be used). This circulator suffers from the following disadvantages: it requires two spatially separated optical paths, and the ports A and C are perpendicular. This means that the circulator will be bulky when a more compact size is desirable.
A more compact circulator is described in U.S. Pat. No. 5,204,771 by Koga; see FIG. 2. This circulator shows an improvement over the previous one since the two optical paths can be in close proximity, and the first and third ports (designated 27 and 28 in the drawing) are parallel. Unfortunately, this device still suffers from a disadvantage. A lens must be placed between the first optical fiber and the circulator to collimate light coming from the first fiber. A lens must also be placed between the third fiber and the circulator to focus light onto the third fiber. If the first and third fibers are far enough apart that there is room to insert two lenses side by side (one for each fiber), the circulator will have to be quite large. Such a circulator will also be expensive, since the cost increases with the size of the components.
If the first and third ports (27 and 28 in FIG. 2) are very close together, the first and third fibers will have to share a common lens for collimating and focusing. However, it is impossible for a single lens to perform adequately for both fibers. The difficulty can be traced to the fact that the light beams coupled to the first and third ports are parallel, and a single lens cannot focus two parallel beams to two different points (i.e., to two different fibers). This prior art therefore suffers from the shortcoming that it cannot be manufactured economically when the circulator is large, and it cannot be efficiently coupled to optical fibers when the circulator is small.